In production of an electric cable, it is common to cover a conductor with an insulating material. As one of methods for producing electric cables, there is a method in which an insulating tape prepared by processing a laminate having a fluorinated resin layer formed on one or both surfaces of a polyimide film, into a tape form, is wound on the surface of a conductor to cover the surface. In such a method, in order to secure insulating properties of the conductor surface, it is usual to heat-seal the fluorinated resin layer after covering the conductor with the insulating tape.
Such an insulating tape is preferably used for aerospace applications, since the polyimide is excellent in properties such as heat resistance, etc. and the fluorinated resin is excellent in electric insulating properties, heat resistance, chemical resistance, weather resistance, etc. Further, in recent years, along with progress in high performance and high functionality of electric equipments, electric cables used for such electric equipments have been likely to be exposed to a high temperature, high humidity environment. Further, there are many cases wherein the amount of current increases, and due to heat generation, the electric cable is likely to be exposed to a higher temperature. The above insulating tape is expected to be useful also for such applications.
In the insulating tape, as the fluorinated resin, a polytetrafluoroethylene (PTFE), a tetrafluoroethylene/hexafluoropropylene copolymer (FEP) or the like is used (see e.g. Patent Documents 1 and 2). Particularly, as the outer-most layer, a PTFE layer is formed in many cases, from the viewpoint of the electrical insulating properties and weather resistance.
As the method for producing such an insulating tape, a method of coating a polyimide film with a dispersion of a fluorinated resin, is common.
However, this method has such a problem that the productivity is no good. For example, in order to bring the fluorinated resin layer to have a certain thickness, it is required to repeat application and drying of the dispersion, which requires time and effort. Further, thermal treatment at a high temperature is required at the time of producing the insulating tape or at the time of producing an electric cable by using the insulating tape. For example, at the time of producing an electric cable by using an insulating tape, usually, the insulating tape is wound spirally on a conductor so that the tape would overlap itself, and then, thermal treatment is conducted to fuse the insulating tape to itself to seal a space in the wound insulating tape. The thermal treatment at that time serves also as a sintering step, and, for example, in the case of PTFE, thermal treatment at a temperature of at least 360° C. will be required after winding an insulating tape provided with a non-sintered PTFE layer, on a conductor. Especially when it is desired to complete the thermal treatment in a short time (e.g. in about one hour), it is required to conduct heating at a temperature of at least 400° C.
As a method for producing a laminate having a fluorinated resin layer, there is also a method wherein a fluorinated resin film and another film are laminated by e.g. thermal lamination or extrusion lamination.
However, an insulating tape obtained by laminating a PTFE film and a polyimide film, has such a problem that the adhesion between the films is low. According to a study by the present inventors, in the case of an insulating tape obtained by laminating a PTFE film and a polyimide film, there is such a problem that even if the above-mentioned thermal treatment is conducted after winding the insulating tape on a conductor, the adhesion between the PTFE film and the polyimide film stays to be low, and peeling is likely to occur between these films during or after the production of an electric cable.
If the thermal treatment temperature is raised, although the adhesion between the PTFE film and the polyimide film may possibly be improved to some extent, the resin, particularly the polyimide having a low heat resistance as compared with PTFE, is likely to be deteriorated, and the production costs will also be increased.
The adhesion of a FEP film to a polyimide film is better than that of a PTFE film, but cannot be said to be adequate, and if the FEP film is laminated on a polyimide film, there will be the same problem as in the case of the PTFE film.